Watershed Hydrology and Water Resources Science Teacher

Transcription

Watershed Hydrology and Water Resources
Science Teacher Education Program
(STEP)
Presented by
Amy Tidwell
Water and Environmental Research Center/
Institute of Northern Engineering
University of Alaska Fairbanks
July 2007
STEP July, 2007: Hydrology – Page No. 1
Watershed Hydrology and Water Resources
Outline
Watershed Hydrology
Watershed Delineation (Exercise)
Water Budget
Precipitation
Evapotranspiration
Infiltration
Runoff
Groundwater (Demonstration/Activity)
Wetlands (Hand out)
Climate Change Considerations
Water Resources
Water Resources Planning and Management (Hand out)
Water Supply
Water as a Hazard
Water Management: Health, Safety, and the Environment
Climate Change and Water Resources
Additional Resources
Online Activities (time permitting)
Watershed Hydrology
Watershed Hydrology
Watersheds
We will apply some of what you’ve learned about the global hydrologic cycle to watersheds. As
you will see, a watershed is a logical accounting unit in hydrology and water resources.
What is a watershed?
Where is a watershed? And how large is a watershed?
Delineating watersheds
• Topographic maps, contour lines and slopes– where does the water flow?
• Begin with a point of interest, usually along a stream.
• Trace the outline of the watershed, beginning at one side of the stream, by following the steepest
slope (gradient). Recall that the steepest gradient occurs at a right angle to contour lines.
• Then begin tracing the outline from the other side of the stream until your second trace meets up
with the first.
• Check your work: Consider a rain drop falling over your delineated watershed. Pick several
points around and even outside of your watershed and trace the downhill flow path of the rain
drop. Is it consistent with your drawn watershed boundary?
USGS, 2001
USGS, 2001
Watershed Hydrology
Hydrologic Cycle and the Water Budget
P
I
Q
ET
G
Where,
P = precipitation
ET = evapotranspiration
I = Infiltration
G=Groundwater
Q= Runoff
ΔWatershed Storage = P – ET – G – R
The Hydrologic Cycle and its Role in Arctic and Global
Environmental Change, 2001.
Precipitation
Precipitation
Introduction
• Precipitation is the primary driver for the land phase of the hydrologic cycle
• Total precipitation over land gets partitioned into different components: some soaks into the
ground (infiltration), some evaporates from the surface of leaves and soil, some is taken up into
plant roots and released back into the atmosphere (transpiration), some is stored at the surface as
snow/ice.
• For a given watershed, how precipitation is partitioned depends on a number of environmental
factors:
– Temperature
– Soil moisture
– Intensity of rainfall
– Vegetation (seasonal effects)
• Furthermore, the state of precipitation (liquid/solid) is very important for seasonal (and
sometimes interannual) partitioning.
• C
USGS, 2001
Precipitation
Mean Areal Precipitation
•Precipitation measurements are point data; however, models often require the amount of rainfall
over the watershed or area of interest.
•As a result, several methods have been developed to determine the average rainfall over the
watershed - called the mean areal precipitation (MAP).
MAP = Total Precipitation Volume
Watershed Area
G4
G3
G2
•Examples of methods include:
– Arithmetic Average (simple average of
all stations)
– Thiessen Polygon (weighted average based
on area of influence)
– Hypsometric (weighted average based on
basin topography and location of stations)
G1
G5
G6
G7
Precipitation
Where to Obtain Precipitation Data (and Other Surface Observations)
• In the United States the National Weather Service (www.nws.noaa.gov) has a
network of precipitation gages
– 278 primary stations - staffed full time by paid technicians (~20 AK)
– 8,000 cooperative stations - mostly volunteer stations (~70 AK)
• Historical data for these stations may be downloaded at the National Climate
Data Center website (www.ncdc.noaa.gov)
Precipitation
Precipitation gage networks
Primary Stations
Dingman, 2002
Precipitation and Rainfall Climatology
Precipitation gage networks
Cooperative Stations
Dingman, 2002
Evapotranspiration
Evapotranspiration
Overview
Evaporation + Transpiration = Evapotranspiration
• Evaporation (E) occurs when water is converted into
water vapor. This may occur from an open water
surface or through exfiltration of soil moisture.
• Methods for estimating evaporation include:
– Water budget
– Energy budget
– Mass transfer techniques
– Pan evaporation measurements
• Transpiration occurs when water vapor is lost to the
atmosphere through small openings in the leaves of
plants.
• Potential Evapotranspiration (PET) is a combined
estimate of the maximum potential evaporation +
transpiration over an area. When there is limited water
(open surface or soil moisture) actual rates of
evapotranspiration (ET) are less than the potential rate.
Transpiration
Transpiration
Translocation
Translocation
soil surface
Absorption
Absorption
Evapotranspiration
Estimating Evaporation: Water Balance Method
Mass Balance for Water body:
ΔV = P + SWin + GWin − E – SWout – GWout
Solve for E
E = P + SWin + GWin – SWout – GWout − ΔV
Where,
E = Evaporation
P = Precipitation
SWin = Surface Water Inflow
SWout = Surface Water Outflow
GWin = Groundwater Inflow
GWout = Groundwater Outflow
ΔV = Change in storage
P
P
E
E
ΔV
ΔV
SW
SWout
out
GW
GWout
out
SW
SWinin
GW
GWinin
• The water balance method is computationally simple. However, gathering the data for
implementation of this method may be difficult.
• Each of the quantities in the equation above are measured or estimated, which results in
uncertainty. Thus the calculation of evaporation includes the sum of the errors related to each
component.
Evapotranspiration
Estimating Evaporation: Penman Method
Δ • ( K + L) + γ • K E • ρ w • λv • va • ea* • (1 − ra )
E=
ρ w • λv • (Δ + γ )
Where,
⎛ 17.3 • Ta ⎞
2508
.
3
⎟
• exp⎜
Δ =
⎜ T + 237.3 ⎟ with Ta in ºC
2
(Ta + 237.3)
⎝ a
⎠
γ = psychrometric constant = (ca • P) (0.622 • λv )
ca =heat capacity of air=1.0x10-3 [MJ/kgK]
P = pressure [kPa]
λv = latent heat of vaporization [MJ/Kg]
= 2.50 – 2.36x10-3 •Ts, T in ºC
K = net short-wave radiation input
= Io • (0.803-0.34c-0.458c2) • (1- a)
Io = solar insolation at the top of the atmosphere
[MJ/m2day]
a = albedo
c = Cloud cover
KE = coefficient reflecting the efficiency of
vertical transport of water vapor by turbulent
eddies of the wind = 1.69 • 10-5 • AL-0.05
AL = water surface area [km2]
ρw = density of water = 1000 [kg/m3]
Required
Required input
input data
data
1.
1. A
ALL –– used
used in
in K
KEE
2.
2. PP or
or Altitude
Altitude
3.
used in
in λλvv and
and LL
3. TTss –– used
4.
4. TTaa –– used
used in
in ΔΔ ,, LL and
and eeaa**
5.
5. eeaa
6.
6. vvaa
7.
7. cc –– used
used in
in K
K
8.
8. IIoo –– used
used in
in K
K
9.
9. aa –– used
used in
in K
K
L = net long wave radiation input
= ε w • ε at • σ • (Ta + 273.4)4 − ε w • σ • (Ts + 273.2)4
Ta = temperature of atmosphere, in ºC
Ts = temperature of surface, in º C
σ=Stefan-Boltzmann const
= 4.90x10-9 [MJ/m2dayK4]
εw= effective emissivity of water = 0.97
εat= effective emissivity of atmosphere
1/ 7
⎛
⎞
ea
⎟⎟ • (1 + 0.22 • C 2 )
= 1.72 • ⎜⎜
⎝ Ta + 273.2 ⎠
va = wind speed [km/day]
ra = relative humidity = ea/e*a
e*a = saturation vapor pressure at the air
temperature = 0.611• exp⎛⎜ 17.3 • Ta ⎞⎟ [kPa]
⎜ T + 237.3 ⎟
⎝ a
⎠
Infiltration
Infiltration
Clay particles
Soil Properties
• The properties of a homogeneous soil matrix
include:
– Porosity, φ =
V
Volume Air & Water
= void
Volume Air,Water & Minerals Vtot
– Water content, θ =
V
Volume Water
= w
Volume Air, Water & Minerals Vt ot
Sand grains
Dingman, 2002
– Field capacity, θfc = water content
at which further drainage due to
gravity is negligible
– Permanent wilting point, θpwp = water
content at which plants are unable to
extract additional water
• If a soil is saturated and then allowed to drain,
its water content will decrease indefinitely in a
quasi-exponential manner, with the drainage rate
negligible within a few days to a week
Dingman, 2002
Infiltration
Hydrologic Horizons
• Ground-water zone: Saturated, positive pressure; in absence of ground-water flow pressure is
hydrostatic p( z ) = γ w ⋅ ( z1 − z 2 ) ; z1 > z2 , where p is the pressure, z is the height above the datum,
and γw is the specific weight of water
• Tension-saturated zone (capillary fringe):
Saturated zone above the water table due
to capillary rise through the pore spaces;
pressure is zero at the top of the water table
and negative in the capillary fringe
• Intermediate zone: Water enters as
percolation from above and leaves by
gravity drainage
• Root zone: Layer from which plant roots can
extract water, bounded by the surface above
and an indefinite and irregular lower bound;
water enters by infiltration and leaves via
transpiration and gravity drainage
Dingman, 2002
Infiltration
The Infiltration Process
• Infiltration is the process by which water arriving at the soil surface enters the soil column. The
maximum rate that a soil can accept water is called the infiltration capacity, f(t)*.
• At a given point the infiltration rate, f(t), changes systematically with time and is influenced by:
– The rate at which water arrives from above, w(t), or the depth of ponding on the surface, H(t)
– The hydraulic conductivity of the soil, Kh*
– Antecedent soil moisture
• Three general conditions during infiltration may be distinguished
– No ponding: In this case the infiltration rate equals the water-input rate and is less than or
equal to the infiltrability
H (t ) = 0, f (t ) = w(t ) ≤ f * (t )
– Saturation from above: Ponding is present because the water-input rate exceeds the
infiltrability in which case the infiltration rate equals the infiltrability
H (t ) > 0, f (t ) = f * (t ) ≤ w(t )
– Saturation from below: Ponding is present because the water table has risen to or above the
surface in which case the infiltration rate is zero
H (t ) ≥ 0, f (t ) = 0
Runoff
Runoff
Basic Aspects of Stream Response
Definitions
• Watershed response to an input event is characterized by
stream discharge at a single point that defines the outlet of
the watershed
Rain (depth/time)
• A graph of water input vs. time can be constructed from
spatially averaged precipitation measurements and is called
a hyetograph
Hyetograph
• A graph of stream discharge vs. time is a streamflow
hydrograph
• A storm hydrograph is the time trace made by an observer
at a fixed point of a flood wave moving downstream
Discharge (volume/time)
Hydrograph
Dingman, 2002
Runoff
Basic Aspects of Stream Response
Streamflow
• Streamflow is a spatially and temporally integrated response
determined by
– Spatially and temporally varying input rates (precipitation, snow
melt, glacial melt)
– Time required for each drop of water to travel from where it
strikes the watershed surface to the stream network (determined
by length, slope, vegetative cover, soils, and geology of
hillslopes)
– Time required for water to travel from its entrance into the
channel to the point of measurement
• Flow may enter the stream at the surface, from overland flow and
channel precipitation, and as subsurface flow, from groundwater and
interflow
• Flow in the stream takes the form of a flood wave that moves
downstream through the stream network
Dingman, 2002
Runoff
Response Hydrographs
Effective Rainfall
• Only a fraction of water input to the watershed actually appears in the response hydrograph, with
the remainder leaving the watershed as
– Evapotranspiration
– Streamflow that is realized too long after
the input event to be associated with that
event (baseflow)
– Groundwater outflow (other than baseflow)
• Depending on the type of model, it is often
necessary to estimate the effective rainfall from
the hyetograph of water input
• There are several approaches used for this
estimation as shown here
– a) Losses equal to a constant fraction of
water input for each time period
– b) Losses equal a constant rate throughout event
Dingman, 2002
– c) Losses given by an initial abstraction followed by a constant rate
– d) Losses given by an approximation to an infiltration-type curve
Runoff
Response Hydrographs
Hydrograph Separation
• Event flow is streamflow resulting from the effective rainfall
• Hydrograph separation divides the hydrograph into a portion attributed to event flow and a
portion attributed to baseflow
• Gaging station measurements of streamflow cannot distinguish event flow from flow originating
from a previous event
• Therefore, graphical hydrograph
separation is often used as a convenient
delineation in order to analyze and
model event responses and the factors
influencing them
• Graphical separation does not
actually identify flow from different
sources
After Linsley and Franzini, 1979
Flow statistics of three rivers near the
headwaters of the Yukon River.
USGS, 2001
Runoff
Where to Obtain Streamflow Data (and Groundwater Data)
US Geological Survey
•http://water.usgs.gov/ Æ Select Alaska Æ Select “Real Time Data Table”
Æ Select station Æ Select data product (Daily data is usually best)
Runoff
Simple Runoff Model
Rational Method
• Regional equations suitable for assessing the impact of developing on peak discharge are not
generally available for small watersheds
• One widely used method, intended for use on small watersheds, is the Rational Method, which
relates the peak discharge of an area, qp(ft3/s), to
– Drainage area, A (acres),
– Rainfall intensity, i (in/hr),
– Runoff coefficient, C
q = CiA
• Rainfall intensity is obtained from an intensity-duration-frequency (IDF) curve using a specified
return period
• Primary use of Rational Method: design problems for small urban areas (small drainage areas,
short times of concentration)
Runoff
Sophisticated Model: Sacramento Soil Moisture Accounting Model
• The model states consist of the contents of various conceptual reservoirs identified in the upper
and lower soil zones
• Water fills and spills over in a cascade of reservoirs based on parameters that represent average
soil characteristics in each reservoir
• This movement of water between compartments is governed by the precipitation rate, the
capacities of each reservoir,
evapotranspiration, and the rates at
which water can transfer between
compartments (infiltration,
interflow, or percolation)
• While an infinite number of layers
could be established, the goal of
parameterization is to use no more
than necessary to effectively describe
the physical system
Runoff
Sophisticated Runoff Model
Groundwater
Groundwater
Basic Groundwater Characteristics
•The groundwater portion of the hydrologic cycle is rather complex
– Water enters at the surface (infiltration),
– Redistributes under forces of gravity, energy gradients, capillary rise, and evapotranspiration
– Water percolates to lower water reservoirs (aquifers)
– Groundwater flows under the influence of energy gradients
– Groundwater may flow into or receive recharge from surface water bodies
– Pumping of groundwater alters the region around the well by drawing down either the water
Pumped
table or the piezometric surface (cone of depression)
Snow
well
In
fil
tra
tio
n
Unsaturated
zone
Perched water table
Percolation
Perched aquifer
Influent stream
(seepage from stream)
Spring
Spring
Water
table
Lake
Cone of
depression
Zone of
saturation
Piezometric
surface
Water
Ground water
flow
Mars
h
table
Effluent stream
(seepage into stream)
Unconfined aquifer
Ocean
Confining layer
Saltwater
intrusion
Confined (artesian)
aquifer
Bedrock
Artesian
well
Reproduced from McCuen, 1998
Groundwater
Basic Groundwater Characteristics
Definitions
•Water that enters the soil is considered soil moisture while in the unsaturated zone and is called
groundwater once in the saturated zone.
•Within the saturated zone water occupies all pore space and is under hydrostatic pressure
•Aquifer- groundwater-bearing formations sufficiently permeable to transmit and yield usable
quantities of water
•Unconfined Aquifer- permeable underground formation having a surface at atmospheric pressure
•Confined Aquifer- confined (or artesian) aquifers form between layers of very low permeability
material
– If the layers are essentially impermeable they are called
aquicludes
– If the layers are permeable to transmit
water vertically to or from the confined
aquifer, but not permeable enough for
lateral transport, they are called
aquitards
Bouwer, 1978
Wetlands
Handout: What are wetlands
and why are they important?
USGS,
2001 No. 38
STEP July, 2007: Hydrology
– Page
Watershed Hydrology
Base Max
Base Min
Base Mean
Future Max
Future Min
Future Mean
30-Year Monthly Mean Flow
6000
Climate Change Considerations
Flow (mcm)
• Ways that climate change might affect
hydrology: (class suggestions- recall the
components of the water budget)
5000
4000
3000
Atbara
2000
1000
0
1
• Example from the Nile basin
2
3
4
5
6
7
8
9
10
11
Base Max
Base Min
Base Mean
Future Max
Future Min
Future Mean
18000
• How are Alaska and the Arctic different
from lower latitudes?
16000
14000
Flow (mcm)
• Evapotranspiration- temperature, soil
moisture, vegetation
12
Month
12000
Blue Nile
10000
8000
6000
4000
2000
• Glacial fed streams
0
1
2
3
4
5
6
7
8
9
10
11
12
Month
• Continuous permafrost regions
Base Max
Base Min
Base Mean
Future Max
Future Min
Future Mean
3000
2500
• Groundwater
Flow (mcm)
• Discontinuous permafrost
Sobat
2000
1500
1000
500
• Storm frequency/intensity
0
1
2
3
4
5
6
7
8
9
10
11
12
Month
USGS, 2001
Water Resources
Water Resources Planning and Management
Introduction to Water Resources
Handout: The development of Dryville
Water Resources Planning and Management
Role of Hydrologic Analysis
Hydrologic analysis for water resources management can be categorized according to the following
assessments:
1. Present and future supply of water available from surface and/or ground water sources;
2. Present and future quality of surface and/or ground water;
3. Present and future frequency with which human activities will be subject to floods; and
4. Present and future frequencies of low streamflows and drought.
Relationships among water resources management goals, purposes, and types of analyses:
Goals
Purpose
Economic
Development
Environmental
Quality
Social WellBeing
Hydrologic
Analysis
Public water supply
X
X
WS, D, Q
Industrial water supply
X
X
WS, D, Q
Irrigation
X
X
WS, D, Q
Hydroelectric power
X
X
(D, F)
WSWS
Navigation
X
X
WS
Waste transport and treatment
X
X
WS, Q
Recreation
X
X
WS, Q
X
(Q, F)
WSWS
X
F
Wildlife habitat
Reduction of flood damages
X
X
X
WS = water supply; D = drought; Q = water quality; F = flood magnitude-frequency.
After Dingman, 2002
Water Supply
Water in Our Communities
• Water Users
• Water Uses
Environmental needsusers, uses, minimum
requirements?
• Infrastructure
• Resource allocation and competing objectives
Water as a Hazard
USGS, 2001
Water as a Hazard
Planning, Management and Risk
There’s not enough…
• Storage: Reservoirs, tanks, groundwater
– Redistribute water for year round availability, according to demand
– Mitigate the effects of drought (period of consecutive dry years)
• Emergency supply: fire flows require greater water volumes and much higher pressures
There’s too much…
• Storms and runoff
• Seasonal high flows
• Excessive wet years
(storage plays an important role here as well)
• Wetlands??
It’s dirty…
• Sediment and mineral loads (water quality, water treatment)
• Naturally occurring and introduced contaminants (suitability, pollution)
• Flooding and water quality (contamination)
Water as a Hazard
Impacts of Water Management
Activity
Magnitude-Frequency
Timing
Duration
Rate of Change
Damming
Reduced variability (WS, FC);
Reduced peak flows (FC)
Altered (WS, FC,
HP)
Reduced periods of
inundation (FC)
Rapid fluctuations
(HP)
Diversion
Reduced flows; Reduced
variability
Altered
Urbanization and drainage
Increased variability; Increased
peak flows
Reduced periods of
floodplain inundation due
to stream entrenchment
Levees and channelization
May increase downstream
peak flows
Reduced periods of
floodplain inundation
Groundw water pumping
Deforestation
Reduced low flows
Increased variability; Increased
peak flows; Reduced low flows
WS = water supply; FC = flood control; HP = hydropower
After Dingman, 2002
Pollution
• Point source versus Non-point source (NPS)
• Types of pollutants:
– Oxygen-demanding material
– Nutrients
– Pathogens
– Suspended solids
– Toxic metals and organic compounds
– Heat
• Typical water quality concerns for types of water resources:
Water
Resource
Precipitation
pH
Dissolved
Solids
X
X
Ground Water
Suspended
Solids
Dissolved
Oxygen
X
Streams
X
X
Lakes
X
X
X
X
Organics and
Petrleum
Compounds
Pathogenic
Organisms
X
X
X
X
X
Excess Heat
X
X
"X" Indicates that a given type of water-quality constituent is typically of concern in a given type of water resource.
After Dingman, 2002
• Pollution management: “control the discharge of pollutants so that water quality is not
degraded to an unacceptable extent below the natural background level.” Davis and Cornwell
• To do so, we need to: Measure pollutants, predict impacts of pollutants, know background
(natural) water quality, decide on acceptable water quality level for a water body.
Pollution
Biological Oxygen Demand (BOD): dissolved oxygen consumed during the oxidation of an
organic compound. Consumption of dissolved oxygen poses a threat to fish and other higher
forms of aquatic life that must have oxygen to live.
Davis and Cornwell, 1998
Climate Change and Water Resources
Potential Impacts of Climate Change
• Tools for understanding potential consequences of change (for better or worse)
– Global Climate Models (GCMs), historical climate records
– Hydrologic Models (& other environmental models)
– Water Resources Systems Models
– Verification requires observations and an understanding of uncertainty
• Climate Change Impact Assessments:
– Climate change and water supply
– Climate change and infrastructure
– Climate change and ecology
– Sustainability
–…
• What are the appropriate time and spatial scales for these assessments?
– How do we experience the change?
– At what scales do we have confidence in projections and models?
• Hand-out: “Climate Change and Wetlands”
Water Science
Additional Resources
The Water Source Books by the Environmental Protection Agency
http://www.epa.gov/safewater/kids/wsb/index.html
Water Science for Schools by the US Geological Survey
http://ga.water.usgs.gov/edu/
Watershed Game at the Bell Museum site
http://www.bellmuseum.org/distancelearning/watershed/watershed2.html
Rivers 2001 by the National Geographic Society
http://www.nationalgeographic.com/geographyaction/rivers/ga22.html
Alaska Wildlife Curriculum Teacher’s Guide by the Alaska Dept of Fish and Game
http://www.wildlife.alaska.gov/index.cfm?adfg=education.awc
References
Davis, M. and D. Cornwell, 1998. Environmental Engineering. 3rd Ed. McGraw-Hill, Boston, 919
pp.
Dingman, L., 2002. Physical Hydrology. 2nd Ed. Prentice Hall, Upper Saddle River, New Jersey,
646 pp.
McCuen, R., 1998. Hydrologic Analysis and Design. 2nd Ed. Prentice Hall, Upper Saddle River,
New Jersey, 814 pp.
Thompson, S., 1999. Water Use, Management, and Planning in the United States. Academic Press,
San Diego, 371.

Watershed Hydrology and Water Resources Science Teacher

Transcription

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